Multi-decade high voltage driver amplifiers are needed for both cable-television (i.e., 10 MHz-1.2 Ghz) and SONET 12.5 Gb/s OC-192 optical modulator transmitter applications. Wide bandwidth and high output voltage operation is achieved by combining broad band circuit topologies such as distributed and Darlington feedback amplifiers with a high speed technology such as SiGe or InP heterojunction bipolar transistors (HBTs). However, high speed technologies are typically characterized by low device breakdown voltages.
In order to satisfy the need for wide bandwidth and high voltage output, conventional approaches often implement a Darlington amplifier pair. A Darlington pair can increase the bandwidth of a traditional common-emitter feedback amplifier by replacing the common-emitter transistor with a Darlington pair. FIG. 1 illustrates a circuit 10 implementing such a conventional Darlington amplifier which includes transistors Q1 and Q2. Bandwidth improvements as much as a factor of two can be obtained. To further increase bandwidth response, high speed semiconductor technologies such as SiGe or InP heterojunction bipolar transistors (HBTs) can be implemented in the Darlington design. However these higher speed device technologies usually have a low breakdown voltage (BVceo), which can limit the maximum voltage swing and delivered output power. FIG. 2 illustrates the maximum output a swing capability of the circuit 10 of FIG. 1.
The conventional Darlington amplifier is noted for having a high gain and wide bandwidth. However, the maximum output voltage swing of the conventional Darlington pair is limited by the difference between the breakdown voltage BVceo and the saturation knee voltage of the Darlington pair defined as Vbe2+Vce1(sat) as shown in FIG. 2. On the negative output voltage swing of the Darlington pair, the transistor Q1 begins to saturate at (Vbe+Vce(sat)), limiting the negative voltage excursion. On the positive output voltage swing, the output voltage does not swing beyond the breakdown voltage BVceo of the transistor Q2 without damaging the device, which limits the positive voltage excursion. For high speed SiGe or InP HBT processes, the breakdown voltage BVceo tends to be low which can limit implementation in broadband laser modulator driver applications, for example.
Referring to FIG. 3, a current versus voltage characteristic curve of the circuit 10 is shown. The current-voltage offset voltage of the circuit 10 is one base to emitter voltage (Vbe) level higher than the common-emitter saturation voltage. The output voltage swing does not exceed the breakdown voltage BVceo of the device technology.
Referring to FIG. 4, a small signal response of the circuit 10 is illustrated. The gain and 3-dB bandwidth of the conventional Darlington amplifier implementation of the circuit 10 are 15.5 dB and 8 GHz, respectively, as illustrated by the curve S21.
It would be desirable to implement an amplifier or device with an enhanced breakdown voltage while still providing wide gain bandwidth capability without incurring the cost of state of the art technologies such as GaN transistors.